This invention relates to a method for preparing diaryl esters of aliphatic acids. Diaryl esters of aliphatic diacids are used as monomers in melt polymerization processes for preparing polyestercarbonates.
Polycarbonates are well known as tough, clear, highly impact resistant thermoplastic resins. Polycarbonates, however, possess relatively high melt viscosity. The polycarbonate of 4,4xe2x80x2-isopropylidenediphenol (bisphenol A polycarbonate), for instance, is a well know engineering molding plastic.
In order to prepare a molded article from polycarbonate, relatively high extrusion and molding temperatures are required. In order to reduce the melt viscosity while also maintaining the desired physical properties, methods including the addition of plasticizers, the incorporation of aliphatic chainstoppers, the reduction of molecular weight, and the preparation of blends of polycarbonate with other polymers have been practiced.
One method of reducing the melt viscosity while maintaining the desired physical properties of a polymer is the incorporation of residues of aliphatic diacids into the backbone of the polymer chain. These residues are commonly referred to as xe2x80x9csoft blocksxe2x80x9d. Diaryl esters of aliphatic diacids are prepared industrially by two methods: 1) Transesterification with phenyl acetate; 2) Reaction of alkanedioyl dichlorides with phenol.
Some new commercial polycarbonate plants synthesize polycarbonate by a transesterification reaction whereby a diester of carbonic acid (e.g., diarylcarbonate) is condensed with a dihydric compound (e.g., bisphenol-A). This reaction is performed without a solvent, and is driven to completion by mixing the reactants under reduced pressure and high temperature with simultaneous distillation of the phenol produced by the reaction. This synthesis technique is commonly referred to as the xe2x80x9cmeltxe2x80x9d technique. Diaryl esters may be used as monomers in melt polymerization processes to prepare polyestercarbonates. The incorporation of the diaryl esters provides reduced melt viscosity in the polymer, while maintaining the desirable properties of the polycarbonate.
Residual acid contaminates generated by the esterification reactions mentioned above are detrimental to a base-catalyzed melt process used to prepare polyestercarbonates in that the rate of the melt polymerization is decreased. Thus, the diaryl ester monomer must be purified. The purification involves multiple recrystallization steps in solvents. The purification steps add considerable cost to the preparation of the diaryl ester monomers.
It would be desirable to prepare diaryl esters of aliphatic diacids that do not require extensive purification before introduction into a melt process. In particular, it would be desirable to prepare diaryl esters of aliphatic di acids that have a sufficiently low level of acids impurities such that the diaryl esters may be introduced into a melt process without requiring purification steps.
In one aspect, the invention relates to a method for preparing diaryl esters suitable for use in a melt transesterification process, comprising the step of reacting a diaryl carbonate and a dicarboxylic acid in the presence or absence of a base, wherein from 2.005 to 2.2 molar equivalents of diaryl carbonate per molar equivalent of dicarboxylic acid are provided. In one embodiment, the diaryl esters contain less than 10 ppm of residual carboxylic acids.
In a further aspect, the invention relates to a method for preparing polyestercarbonate by the melt process utilizing the diaryl esters prepared by reacting a diaryl carbonate and a dicarboxylic acid, wherein from 2.01 to 2.02 molar equivalents of diaryl carbonate per molar equivalent of dicarboxylic acid are provided.
The present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention and the examples included therein.
Before the present compositions of matter and methods are disclosed and described, it is to be understood that this invention is not limited to specific synthetic methods or to particular formulations, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting.
In the following specification, reference will be made to a number of terms that shall be defined to have the following meanings:
The singular forms xe2x80x9caxe2x80x9d, xe2x80x9canxe2x80x9d and xe2x80x9cthexe2x80x9d include plural referents unless the context clearly dictates otherwise.
xe2x80x9cOptionalxe2x80x9d or xe2x80x9coptionallyxe2x80x9d means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not.
As used herein, the term xe2x80x9cmelt polycarbonatexe2x80x9d refers to a polycarbonate made by the transesterification of a carbonate diester with a dihydroxy compound.
xe2x80x9cBPAxe2x80x9d is herein defined as bisphenol A or 2,2-bis(4-hydroxyphenyl)propane.
xe2x80x9cSBIxe2x80x9d is herein defined as 6,6xe2x80x2-dihydroxy-3,3,3xe2x80x2,3xe2x80x2-tetramethylspirobiindane.
xe2x80x9cCD-1xe2x80x9d is herein defined as 6-hydroxy-1-(4xe2x80x2-hydroxyphenyl)-1,3,3-trimethylindane.
xe2x80x9cBCCxe2x80x9d is herein defined as 1,1-bis(4-hydroxy-3-methylphenyl)cyclohexane
xe2x80x9cDPCxe2x80x9d as used herein is defined as diphenyl carbonate.
The terms xe2x80x9cdiphenolxe2x80x9d and xe2x80x9cdihydric phenolxe2x80x9d as used herein are synonymous.
I. Preparation of Diarylesters
In one aspect, the present invention relates to a method of preparing diaryl esters of aliphatic diacids. The diaryl esters are prepared by the transesterification reaction of a diarylcarbonate and an aliphatic diacid.
The diaryl carbonate is a carbonate diester of an aromatic monohydroxy compound, which is represented by formula (I): 
wherein each of Ar1 and Ar2 each independently represents a monovalent carbocyclic or heterocyclic aromatic group; preferably having from 5 to 12 carbon atoms.
Each of the monovalent aromatic groups Ar1 and Ar2 may be unsubstituted or substituted with at least one substituent which does not adversely affect the reaction. Examples of such substituents include, but are not limited to, a halogen atom, a lower alkyl group, a lower alkoxy group, a phenyl group, a phenoxy group, a vinyl group, a cyano group, an ester group, an amide group and a nitro group. Representative examples of monovalent aromatic groups include a phenyl group, a napthyl group, a biphenyl group and a pyridyl group, each of which is substituted or unsubstituted with at least one substituent, as mentioned above.
Representative examples of diaryl carbonates include diphenyl carbonates represented by formula (II) 
wherein each of R1 and R2 independently represents a hydrogen atom; a halogen atom; a lower alkyl group; a lower alkoxy group; a cycloalkyl group having from 5 to 10 ring carbon atoms or a phenyl group, and each of m an n independently represents an integer of from 1 to 5; with the proviso that when m is an integer of from 2 to 5, each R1 may be the same or different; and when n is an integer from 2 to 5, each R2 may be the same or different.
Of the diphenyl carbonates represented by formula (II), it is preferable that the diphenyl carbonates have a symmetrical structure; such as diphenyl carbonate; ditolyl carbonate and diphenyl carbonate substituted with a lower alkyl group having from 1 to 4 carbon atoms. In one embodiment, diphenyl carbonate, having formula (III) is used. 
In the method of the present invention, a transesterification reaction between the diaryl carbonate and an aliphatic diacid, also referred to herein as a xe2x80x9cdicarboxylic acidxe2x80x9d is conducted to produce diaryl esters. The aliphatic diacid is represented by the general formula (IV): 
wherein R3 is a C1-C40 branched or unbranched alkyl or branched or unbranched cycloalkyl.
Representative acids of structure (IV) include, but are not limited, to dodecanedioic acid, sebacic acid, adipic acid, octadecanedioic acid, octadec-9 enedioic acid, 9-carboyxoctadecanoic acid and 10-carboxyoctadecanoic acid. Dodecanedioic acid (DDDA) is the more preferred.
The present invention provides a process whereby diaryl esters are produced having suitable purity for introduction into a subsequent melt polymerization to prepare polyestercarbonates, without requiring extensive purification. In particular, diaryl esters are produced having less than about 0.01 wt percent of free carboxylic acids, based upon the weight of the reaction product and assuming that no phenol is distilled. It is preferable that the product reaction mixture contain less than about 500 ppm residual acid; more preferably less than 100 ppm residual acid; even more preferably less than about 10 ppm residual acid.
In order to obtain a reaction product having the specifications identified above, the reaction is conducted with a stoichiometric excess of diarylcarbonate; in particular from about 2.001 to about 3, more preferably 2.005 to 2.2, even more preferably from about 2.005 to about 2.1 molar equivalents of diaryl carbonate per molar equivalent of dicarboxylic acid are provided during the course of the reaction. In the present invention, it is undesirable to have residual carboxylic acid groups in the product. Residual carboxylic acid groups interfere with the base catalyzed process to prepare polyestercarbonates.
The reaction of the carboxyl groups of the dicarboxylic acid with the diarylcarbonate is optionally accompanied by the elimination of the aromatic hydroxy compound from which the diarylcarbonate was formed, and carbon dioxide. To remove the hydroxy compounds from the reaction mixture, it is heated at reduced pressure, after the reaction is underway. Alternatively, the aromatic compound may remain in the reaction, where it is eventually removed in a wash step, with methanol, for example.
The reaction may be conducted in the presence or the absence of a base catalyst. If a base catalyst is used, the reaction proceeds at a faster rate, however the reaction product will have more impurities. Suitable base catalysts include, but are not limited to tertiary amines such as triethylamine, quaternary phosphonium compounds such as tetraphenylphosphonium tertaphenylborate, quaternary ammonium compounds such as tetramethylammonium hydroxide, hexaalkyl guanidinium halides such as hexaethyl guanidium chloride, alkali or alkaline earth metal hydroxides, and the like. In typical embodiments quaternary ammonium hydroxides are used a preferred base. The base is preferably used in an amount less than 2xc3x9710xe2x88x924 molar equivalents, based on the aliphatic dicarboxylic acid. If the reaction is conducted in the absence of a base, the process is conducted in an identical manner. The rate of reaction is slower, however the product will have a higher purity.
The reaction is typically conducted by combining the diarylcarbonate and the dicarboxylic acid in the aforementioned ratios, and optionally the base catalyst, at a temperature at from about 180xc2x0 C. to about 220xc2x0 C., preferably at about 200xc2x0 C. Vigorous evolution of carbon dioxide ensues after all components have melted. Optionally, a moderate vacuum is applied to distill the monohydroxyaromatic compound, for example phenol, from the melt.
In an alternative embodiment, the transesterification may be conducted in a solvent. Any inert solvent having a boiling point above about 120xc2x0 C., more preferably above about 180xc2x0 C. is suitable. Examples of suitable solvents include, but are not limited to phenols; polyethylene glycols; polyaromatic compounds; hydrocarbon oils and mixtures thereof. In one embodiment, phenol is used as a solvent as it is already created during the transesterification reaction and can be easily removed by precipitation or distillation. The solvent may comprise from about 0.1 to about 10 parts by weight, of the initial reaction composition, based on the total weight of the reaction composition; more preferably from about 1 to about 3 parts by weight of the initial reaction composition.
The reaction can be conducted as a batch or a continuous process. Any desired apparatus can be used for the reaction. The material and the structure of the reactor used in the present invention is not particularly limited as long as the reactor has an ordinary capability of stirring It is preferable to have an inert gas supply and vacuum.
Optionally, after the transesterification of the diaryl carbonate and the dicarboxylic acid, the reaction product may be further purified to remove residual diaryl carbonate and acid. In one embodiment, the reaction product is dissolved in from about 1 to about 3 weight equivalents of an organic solvent. Suitable organic solvents include, but are not limited to methanol; and a methanol/methylene chloride solution.
The reaction mixture is typically cooled to from about 40 to about 60xc2x0 C. and the organic solvent is added . The mixture is typically heated up to about 60xc2x0 C. until no solids are observed. Thereafter, the solution is chilled to a temperature of from about 0 to about 5xc2x0 C. which results in the precipitation of crystals of the diaryl ester of the diacids. The precipitate may be filtered and washed at ambient temperature with an organic solvent, for example methanol, to obtain the product as a white flaky solid.
The diaryl ester may be used in the same plant after being made, stored for later use, or packaged for transport, all in commercial quantities. In one embodiment, the diaryl ester is introduced directly into a melt process for the preparation of polyestercarbonate.
II. Melt Process Using Diarylesters as Monomers
In a second aspect, the invention relates to the use of the diarylesters prepared according to the method outlined in section I of the specification in a melt polymerization process in which dihydric phenol and a diester of carbonic acid are reacted along with the diarylester, which is incorporated into the backbone of the polymer. Residues of dihydric phenols which are useful in preparing the polyestercarbonate the invention may be represented by the general formula (V): 
wherein:
R4 is independently selected from halogen, monovalent hydrocarbon, and monovalent hydrocarbonoxy radicals;
R5 is independently selected from halogen, monovalent hydrocarbon, and monovalent hydrocarbonoxy radicals:
W is selected from divalent hydrocarbon radicals, 
n and n1 are independently selected from integers having a value of from 0 to 4 inclusive; and
b is either zero or one.
The monovalent hydrocarbon radicals represented by R4 and R5 include the alkyl, cycloalkyl, aryl, aralkyl and alkaryl radicals. The preferred alkyl radicals are those containing from 1 to about 12 carbon atoms. The preferred cycloalkyl radicals are those containing from 4 to about 8 ring carbon atoms. The preferred aryl radicals are those containing from 6 to 12 ring carbon atoms, i.e., phenyl, naphthyl, and biphenyl. The preferred alkaryl and aralkyl radicals are those containing from 7 to about 14 carbon atoms.
The preferred halogen radicals represented by R4 and R5 are chlorine and bromine.
The divalent hydrocarbon radicals represented by include the alkylene, alkylidene, cycloalkylene and cycloalkylidene radicals. The preferred alkylene radicals are those containing from 2 to about 30 carbon atoms. The preferred alkylidene radicals are those containing from 1 to about 30 carbon atoms. The preferred cycloalkylene and cycloalkylidene radicals are those containing from 6 to about 16 ring carbon atoms.
The monovalent hydrocarbonoxy radicals represented by may be represented by the formulaxe2x80x94OR2 wherein R2 is a monovalent hydrocarbon radical of the type described hereinafore. Preferred monovalent hydrocarbonoxy radicals are the alkoxy and aryloxy radicals.
Suitable dihydric phenols include, but are not limited to, BPA; 2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane; 2,2-bis(3,5-dimethyl-4-hydroxyphenyl)propane; 1,1-bis(4-hydroxyphenyl)cyclohexane; 1,1-bis(3,5-dimethyl-4-hydroxyphenyl)cyclohexane; 1,1-bis(4-hydroxyphenyl)decane; 1,1-bis(4-hydroxyphenyl)propane; 1,1-bis(4-hydroxyphenyl)cyclodecane; 1,1-bis(3,5-dimethyl-4-hydroxyphenyl)cyclododecane; 4,4-dihydroxyphenyl ether; 4,4-thiodiphenol; 4-4-dihydroxy-3,3-dichlorodiaryl ether; 4,4-thiodiphenol; 4,4-dihydroxy-3,3-dichlorodiaryl ether; 4,4-dihydroxy-2,5-dihydroxydiaryl ether; BPI; 1,1-bis(4-hydroxyphenyl)-1-phenylethane; 1,1-bis(3-methyl-4-hydroxyphenyl)-1-phenylethane, and mixtures thereof. In one embodiment, the residues of dihydric phenol in the polycarbonate comprise 100 mol % of residues derived from BPA.
Optionally, polyfunctional compounds may be utilized. Suitable polyfunctional compounds used in the polymerization of branched polycarbonate include, but are not limited to,
1,1,1-tris(4-hydroxyphenyl)ethane,
4-[4-[1,1-bis(4-hydroxyphenyl)-ethyl]-dimethylbennzyl],
trimellitic anhydride,
trimellitic acid, or their acid chloride derivatives.
It is also possible to employ residues of two or more different dihydric phenols having the general structure (V) defined above to form co- and terpolymers. In addition to the residues of dihydric phenols having the formula (V) as defined above, residues of dihydric phenols having formulas (VI), (VII), (VIII) or a mixture thereof may be used to prepared co- or terpolymers. 
where R6, R7, R10 and R11 are independently C1-C6 alkyl,
R8 and R9 are independently H or C1-C5 alkyl,
R12 is H or C1-C3 alkyl and n is 0, 1 or 2; 
where R13,R14 and R16 independently represent C1-C6 alkyl,
R15 is H or C1-C3 alkyl and n is 0, 1 or 2,
R17 is H or C1-C5 alkyl; and 
where R18 and R19 are independently selected from the group consisting of C1-C6 alkyl;
X represents CH2;
m is an integer from 4 to 7;
n is an integer from 1 to 4; and
p is an integer from 1 to 4
with the proviso that at least one of R18 and R19 is in the 3 or 3xe2x80x2 position.
Representative units of structure (VI), include, but are not limited to residues of 6,6xe2x80x2-dihydroxy-3,3,3xe2x80x2,3xe2x80x2-tetramethyl spirobiindane(SBI);6,6xe2x80x2-dihydroxy-3,3,5,3xe2x80x2,3xe2x80x2,5xe2x80x2-hexamethyl spirobiindane;6,6xe2x80x2-dihydroxy-3,3,5,7,3xe2x80x2,3xe2x80x2, 5xe2x80x2,7xe2x80x2-octamethylspirobiindane; 5,5xe2x80x2-diethyl-6,6xe2x80x2-dihydroxy 3,3,3xe2x80x2,3xe2x80x2-tetramethylspirobiindane (diethyl SBI) and mixtures thereof. Residues of SBI and its ortho alkylated homologs, and diethyl SBI are most preferred as component (VI).
Representative units of structure (VII) include, but are not limited to residues of 6-hydroxy-1-(4xe2x80x2-hydroxyphenyl)-1,3,3-trimethylindane (CD-1); 6-hydroxy-1-(4xe2x80x2-hydroxy-3xe2x80x2-methylphenyl)-1,3,3,5-tetramethylindane. Residues of CD-1 are most preferred.
Representative units of structure (VIII) include, but are not limited to residues 1,1-bis(4-hydroxy-3-methylphenyl)cyclohexane (B CC); 1,1-bis(4-hydroxy-3-methylphenyl)cyclopentane; 1,1-bis(4-hydroxy-3-methylphenyl)cycloheptane and mixtures thereof. Residues of BCC (structure IX) are most preferred. 
As the diester of carbonic acid, various compounds may be used, including, but not limited to diaryl carbonate compounds, dialkyl carbonate compounds and alkylaryl carbonate compounds. Suitable diesters of carbonic acid include, but are not limited to, diaryl carbonate; bis(4-t-butylphenyl)carbonate; bis(2,4-dichlorophenyl)carbonate; bis(2,4,6-trichlorphenyl)carbonate; bis(2-cyanophenyl)carbonate; bis(o-nitrophenyl)carbonate; ditolyl carbonate; m-cresol carbonate; dinaphthyl carbonate; bis(diaryl)carbonate; diethylcarbonate; dimethyl carbonate; dibutyl carbonate; dicyclohexyl carbonate; and mixtures thereof. Of these, diaryl carbonate is preferred. If two or more of these compound are utilized, it is preferable that one is diaryl carbonate.
Diphenyl carbonates used to make the polyestercarbonates by melt: Suitable carbonate sources, catalysts and reaction conditions are found in U.S. Pat. No. 5,880,248, and Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, Volume 19, pp. 585-600, herein incorporated by reference.
In the process of the present invention, an endcapping agent may optionally be used. Suitable endcapping agents include monovalent aromatic hydroxy compounds, haloformate derivatives of monovalent aromatic hydroxy compounds, monovalent carboxylic acids, halide derivatives of monovalent carboxylic acids, and mixtures thereof.
Suitable endcapping agents include, but are not limited to phenol, p-tert-butylphenol; p-cumylphenol; p-cumylphenolcarbonate; undecanoic acid, lauric acid, stearic acid; phenyl chloroformate, t-butyl phenyl chloroformate, p-cumyl chloroformate, chroman chloroformate, hydrocardanol, nonyl phenol, octyl phenol; nonyl phenyl chloroformate or a mixture thereof. Furthermore, mixed carbonates and esters composed of endcappers from the list above along with phenol or alkylsalicylates are acceptable.
If present, the endcapping agent is preferably present in amounts of about 0.01 to about 0.20 moles, preferably about 0.02 to about 0.15 moles, even more preferably about 0.02 to about 0.10 moles per 1 mole of the dihydric phenol.
Typical catalysts employed in the melt condensation polymerization process include, but are not limited to, alkali metal compounds, alkaline earth metal compounds, quaternary ammonium compounds, quaternary phosphonium compounds and combinations thereof.
Useful alkali metal compounds as catalysts include sodium hydroxide, potassium hydroxide, lithium hydroxide, sodium hydrogencarbonate, potassium hydrogencarbonate, lithium hydrogencarbonate, sodium carbonate, potassium carbonate, lithium carbonate, sodium acetate, potassium acetate, lithium acetate, sodium stearate, potassium stearate, lithium stearate, sodium borohydride, lithium borohydride, sodium borophenolate, sodium benzoate, potassium benzoate, lithium benzoate, disodium hydrogenphosphate, dipotassium hydrogenphosphate, dilithium hydrogenphosphate, disodium, dipotassium and dilithium salts of BPA and sodium, potassium, and lithium salts of phenol.
Useful alkaline earth metal compounds as catalysts include calcium hydroxide, barium hydroxide, magnesium hydroxide, strontium hydroxide, calcium hydrogen carbonate, barium hydrogencarbonate, magnesium hydrogencarbonate, strontium hydrogencarbonate, calcium carbonate, barium carbonate, magnesium carbonate, strontium carbonate, calcium acetate, barium acetate, magnesium acetate, strontium acetate, calcium stearate, barium stearate, magnesium stearate, and strontium stearate.
Useful quaternary ammonium compounds as catalysts include tetraalkylammonium compounds such as tetramethylammonium hydroxide and tetraethylammonium hydroxide. Examples of quaternary ammonium compounds include, but are not limited to, tetramethylammonium hydroxide (TMAH); tetraethylammonium hydroxide; tetrabutylammonium hydroxide;
trimethylbenzylammonium hydroxide and mixtures thereof. Examples of suitable quaternary phosphonium compounds include, but are not limited to, tetramethylphosphonium hydroxide; tetraethylphosphonium hydroxide; tetrabutylphosphonium hydroxide and mixtures thereof.
Preferred catalysts include tetramethylammonium hydroxide, sodium hydroxide and mixtures thereof.
The preferred polyestercarbonates have a weight average molecular weight of about 5,000 to about 100,000, more preferably of about 10,000 to about 65,000, and most preferably about 18,000 to about 36,000 as measured by gel permeation chromatography versus polystyrene standards.
The reaction conditions of the melt polymerization are not particularly limited and may be conducted in a wide range of operating conditions. The reaction temperature is typically in the range of about 100 to about 350xc2x0 C., more preferably about 180 to about 310xc2x0 C. The pressure may be at atmospheric, or at an added pressure of from atmospheric to about 15 torr in the initial stages of the reaction, and at a reduced pressure at later stages, for example in the range of about 0.2 to about 15 torr. The reaction time is generally about 0.1 hours to about 10 hours.
The melt polymerization may be accomplished in one or more stages, as is known in the art. In one embodiment, the process is conducted as a two stage process. In the two stage process, the first stage is an oligomerization stage, and the second stage is a polymerization stage. In the first stage of this embodiment, the base, for example the quaternary ammonium or phosphonium compound, is introduced into the reaction system comprising the dihydroxy compound and the carbonic acid diester and the diarylester. The first stage is conducted at a temperature of 290xc2x0 C. or lower, preferably 150 to 290xc2x0 C., more preferably 200 to 280xc2x0 C. The duration of the first stage is preferably 0 to 5 hours, even more preferably 0 to 3 hours at a pressure from atmospheric pressure to 100 torr, with a nitrogen atmosphere preferred. Alternatively, the base may be introduced prior to the first stage, in a monomer mix tank, for instance. The contents from the monomer mix tank are fed to the first stage., or anywhere in between. The molecular weight of the oligomer is less than 8,000 Mn.
The base catalyst is typically present in a range between about 10xe2x88x928 moles and about 10xe2x88x923 moles to moles of aromatic dihydroxy compound. In another embodiment, the catalyst is present in a range between about 10xe2x88x927 moles and about 10xe2x88x925 moles to moles of aromatic dihydroxy compound.
Additives may also be added to the polycarbonate product as long as they do not adversely affect the properties of the product. These additives include a wide range of substances that are conventionally added to the polycarbonates for a variety of purposes. Specific examples include heat stabilizers, epoxy compounds, ultraviolet absorbers, mold release agents, colorants, antistatic agents, slipping agents, anti-blocking agents, lubricants, antifogging agents, natural oils, synthetic oils, waxes, organic fillers, flame retardants, inorganic fillers and any other commonly known class of additives.
The reaction can be conducted as a batch or a continuous process. Any desired apparatus can be used for the reaction. The material and the structure of the reactor used in the present invention is not particularly limited as long as the reactor has an ordinary capability of stirring. It is preferable that the reactor is capable of stirring in high viscosity conditions as the viscosity of the reaction system is increased in later stages of the reaction.